Learning constructors and objects

I just had a quiz (luckily no a test) on constructors and objects which went relatively badly.

I thought I had it figured out, but I just don't.

The question we were given was basically to make a sub sandwich simulator. We were just told that it has "bread" "number of vegetables" and "type of meat"

The problem here is genuinely that I do not understand this, and not that I haven't read countlessly about it. So I am reaching out to see if somebody here can actually explain this to me in a way I can understand. It seems like there are just too many methods for what we're trying to do for one.

What I'm wondering is what exactly are the 'modify' methods for? How should I use them?

I am sorry if this seems extremely vague, I thought it was, and maybe I just don't understand it enough. This is the template we were given, but I have filled in a few things that I think I understood. I tried asking questions and taking data in the main method, but I can't use non-static methods to return to the main method.

The main thing I am curious about it the modification methods and how exactly I access them are use them?

Now, you don't understand setters? Try replacing the constructor bread = b with setBread(b). In other words, the setter methods are more or less subsets of operations done by the constructor. It sets values, nothing complex about that.

Now, this isn't exactly modeling a real world sandwich, but if you want to switch the bread for one particular sandwich